1. Field of the Invention
The present invention relates to an improved high-frequency heating apparatus such as a microwave oven for heating foods or liquids by what is called dielectric heating, or in particular, to an improved high-frequency heating apparatus comprising an inverter using a semiconductor switch such as a transistor for generating high-frequency power to supply high-voltage power and heater power to a magnetron.
2. Description of the Related Art
High-frequency heating apparatuses of the above-mentioned type have so far been suggested in various configurations for reducing the size, weight and cost of a power transformer used therewith.
FIG. 1 is a circuit diagram of a conventional high-frequency heating apparatus.
In FIG. 1, a commercial power supply 1, a diode bridge 2 and a capacitor 3 make up a power supply 5 of an inverter 4. The inverter 4, in turn, includes a reset inductor 6, a thyristor 7, a diode 8 and a resonance capacitor 9. The thyristor 7 is adapted to be triggered at a predetermined frequency f.sub.0 by an inverter control circuit 10, with the result that an inverter of the relaxation oscillation type made up of the reset inductor 6 and a series resonance circuit including the primary winding 12 of a boosting transformer 11 and the resonance capacitor 9 is energized at the operating frequency f.sub.0 thereby generating high-voltage power P.sub.0 and heater power P.sub.H respectively in the high-voltage secondary winding 13 of the boosting transformer 11 and the heater winding 14. The high-voltage power P.sub.0 generated in the high-voltage secondary winding 13 is rectified by high-voltage diodes 15, 16 and capacitors 17, 18 and supplied to a magnetron 19. Also, the heater winding 14 makes up a resonance circuit with a capacitor 20, through which the heater power P.sub.H is supplied to the cathode heater of the magnetron 19. Numeral 21 designates a start control circuit for controlling the inverter control circuit 10 for a predetermined time during starting of the inverter 4 thereby reducing the trigger frequency f.sub.0 thereof. This operation is in order to keep low the on-load voltage generated in the high-voltage secondary winding 13 before the cathode of the magnetron 19 is heated at the start time.
FIG. 2 is a diagram showing changes in the high-voltage power P.sub.0, the heater power P.sub.H and the anode voltage V.sub.AKO of the magnetron 19 under no load at the operating frequency f.sub.0 of the inverter 4. When f.sub.0 is a predetermined steady frequency f.sub.01, P.sub.0 and P.sub.H assume respective rated values of 1 KW and 40 W. When the inverter 4 is started with f.sub.0 for starting the apparatus, the no-load anode voltage V.sub.AKO reaches a value as high as 20 KV or more, thereby making difficult the treatment for dielectric strength both technically and in respect of the production cost. For this reason, the inverter control circuit 10 is controlled by a start control circuit 21 in a manner to reduce f.sub.0 to f.sub.0S for a predetermined length of time during starting. When f.sub.0 is equal to f.sub.0S, it is possible to reduce V.sub.AKO to a value lower than 10 KV. The value of P.sub.H, on the other hand, is not reduced greatly but to about 30 W due to the resonance effect of the capacitor 20 included in the heater circuit. As a result, although there is a longer time required before complete heating of the cathode than when the rating of P.sub.H =40 W is involved, there is no abnormally high V.sub.AKO generated in starting the high frequency heating apparatus.
FIGS. 3A, 3B and 3C are diagrams showing the manner in which the operating frequency f.sub.0, the anode voltage V.sub.AK of the magnetron and the anode current I.sub.A of this high-frequency heating apparatus undergo a change during the starting process.
As shown in FIG. 3A, the inverter control circuit 10 is controlled by the start control circuit 21 in such a way that f.sub.0 is controlled to f.sub.0S during the period of time from t=0 to t=t'.sub.1 after which f.sub.0=f.sub.01 holds at time t.sub.2. As a result, as shown in FIG. 3B, the voltage V.sub.AK is regulated as V.sub.AKOmax &lt;10 KV, and as shown in FIG. 3C, the anode current I.sub.A starts and reaches I.sub.Al during the time between t.sub.1 and t.sub.2 thereby producing a rated high voltage output P.sub.0 =1 KW. Specifically, this apparatus is so configured that after the transient period of the region B through a preheating period of the region A, the steady state of the region C is reached.
In this way, the frequency f.sub.0 is reduced to f.sub.0S at the time of starting in a manner compatible with the resonance of the capacitor 20 in the heater circuit, thereby preventing an abnormal high voltage from being generated at the time of first starting. It is thus possible to realize a high-frequency heating apparatus that can be started stably.
This conventional high-frequency heating apparatus, however, has the disadvantages mentioned below.
The heater power P.sub.H is supplied from a heating winding 14 wound on the same core as the high-voltage secondary winding 13 for producing a high voltage power P.sub.0. Therefore, as shown in FIG. 2, it is difficult to maintain P.sub.H constant against the frequency f.sub.0, and even with the provision of a resonance capacitor 20, what can be expected is not more than preventing the value P.sub.H from changing in proportion to P.sub.0, thus attaining at most the characteristic shown by the dashed curve. Specifically, it is impossible to realize more than attaining a P.sub.H of 30 W when f.sub.0 is reduced to f.sub.0S.
FIG. 4 is a diagram showing an example of the relationship between the heater power P.sub.H and the time before start of oscillation of the magnetron after the heater power P.sub.H is supplied to heat the cathode sufficiently, that is, the oscillation start time t.sub.s. As seen from this diagram, in the prior art, it is possible to prevent generation of an abnormally high voltage but it is difficult to supply sufficient heater power P.sub.H during the starting process, so that the oscillation start time t.sub.s is increased to several times longer than when the rated P.sub.H (=40 W) is supplied.
Specifically, the region A shown in FIG. 3C is lengthened, with the result that an application of the prior art circuit to a high-frequency heating apparatus, such as a microwave oven featuring quick cooking in the order of seconds, would unavoidably lead to a reduced material function.
In FIG. 5A, the period of time t from t.sub.1 to t.sub.2 is one where the heater power P.sub.H is gradually increased while the high-voltage power P.sub.0 to the magnetron (that is, the anode current I.sub.A) is increased in the manner shown in FIG. 5C.
FIGS. 5A, 5B and 5C are diagrams showing a relationship in which the heater power P.sub.H, cathode temperature T.sub.C and high-voltage power P.sub.0 increase with the increase in f.sub.0 from f.sub.0S to f.sub.01. As obvious from these diagrams, the cathode temperature T.sub.C which has a predetermined thermal time constant is delayed by .tau. behind the increase in P.sub.H, and reaches a rated temperature when t is t.sub.3. The power P.sub.0, on the other hand, increases at the same time as P.sub.H, and therefore the period involved, that is, from t.sub.1 to t.sub.3 is one in which the cathode is liable to exhibit a phenomenon wherein it is be short of emission or the like. The fact that this region is long results in a very significant disadvantage in that the service life of the cathode of the magnetron is greatly reduced.
Further, to configure a resonance circuit including a capacitor 20 in the heater circuit of the magnetron 19 is very inconvenient in view of the small cathode impedance and the high potential thereof.